n. — proprietary
a folder made of card stock or plastic in various colours, with three fasteners to hold down loose-leaf paper (see Image 1).
Type: 5. Frequency — Duo-Tang is an American product. The company was founded in 1931 and is headquartered in Paw Paw, Michigan (Bloomberg Business website). As Chart 1 shows, duo-tang is not widely used in the US, but is very frequent in Canada.
See also COD-2, s.v. "Duo-Tang", which is marked "Cdn proprietary".
See also: exercise book
- Combinations exist in terms such as Duotang-bound book (see the 1987 quotation) or duo-tang folder (see the 2006 quotation).
Images:

Chart 1: Internet Domain Search, 19 Jul. 2012 
Image 1: Duo-tang folders. Photo: S. Gaylie.